The present invention relates to a process for regenerating high-purity sulfuric acid having increased oxidizing power from waste sulfuric acid discharged from a surface treatment or resist stripping step involved in the process of fabricating semiconductor devices such as LSIs and VLSIs or liquid crystal display devices, and recycling it.
Sulfuric acid has a wide spectrum of applications. Sulfuric acid effluents discharged from plants, etc., have a decreased sulfuric acid content and contain salts, and so they are concentrated by water removal and cleared of salts for regeneration.
The regeneration of waste sulfuric acid is achieved by:
(1) a vacuum concentration process wherein waste sulfuric acid is heated under reduced pressure to evaporate water and the deposited salts are separated from sulfuric acid by crystallization, PA1 (2) a cooling process wherein waste sulfuric acid is cooled to crystallize out salts due to a solubility drop for sulfuric acid recovery, PA1 (3) a vacuum cooling concentration wherein waste sulfuric acid is thermally concentrated under reduced pressure and the concentrate is cooled for crystallization and separation, PA1 (4) a submerged combustion process wherein waste sulfuric acid is concentrated by submerged combustion, while salts are crystallized out for separation, thereby recovering sulfuric acid, PA1 (5) a solvent extraction process wherein salts, organic materials, etc. are extracted and removed from waste sulfuric acid using acetyl acetone, benzene, etc. as a solvent and making use of a solubility difference therebetween, PA1 (6) a pyrolysis process wherein waste sulfuric acid is decomposed into sulfur oxides in a pyrolysis furnace, and the sulfur oxides are absorbed in water or sulfuric acid for the recovery of sulfuric acid, PA1 (7) a diffusive dialysis process wherein waste sulfuric acid flows in countercurrent relation to water through an anion exchange membrane to pass sulfuric acid into water by diffusion due to a temperature difference and the selective permeation of the anion exchange membrane, thereby recovering the sulfuric acid, and PA1 (8) a two-stage distillation process wherein waste sulfuric acid is heated at a temperature not higher than 300.degree. C. to remove a substantial part of organic matter and water, and the resulting sulfuric acid is distilled at a temperature not lower than 300.degree. C. to separate sulfuric acid from salts and high-boiling compounds for the recovery of sulfuric acid.
With the tendency of semiconductor devices to becoming finer and having higher density, a severer restriction is now imposed on the purity of sulfuric acid for electronics industry. For instance, the sulfuric acid is required to have a metallic component content of at most 20 ppb. However, the processes (1) to (5) mentioned above are all applied to the recovery of waste sulfuric acid discharged in large amounts and on an industrial scale from viscose rayon, petroleum purification, anodized aluminum and pickling factories or plants. With these methods it is impossible to obtain high-purity sulfuric acid thanks to incomplete removal of salts. In other words, the sulfuric acid recovered by these methods have application in some fields in which sulfuric acid of high purity is not needed.
According to the diffusive dialysis process (8) it is possible to recover sulfuric acid of relatively high purity. However, the obtained sulfuric acid cannot immediately be used thanks to its low concentration. On the other hand, problems with the pyrolysis (6) and two-stage distillation (8) processes are that they are hazardous to personnel around the equipment or incurs some considerable maintenance expense due to equipment corrosion or for other reasons, because sulfuric acid is pyrolyzed, distilled or otherwise handled at high temperature.
Further, sulfuric acid for electronics industry--which is used for fabricating semiconductor devices, etc.--is mixed with a hydrogen peroxide solution for use, because it is required to increase the force with which photoresists are stripped or washed. According to the conventional processes, however, sulfuric acid is merely recovered; that is, no oxidizing substance is generated in sulfuric acid. It is thus required that fresh hydrogen peroxide solutions be supplied to the equipment during use.
The inventors have already filed a patent application for a process for recovering sulfuric acid--which is of purity high-enough to be reused at the electronics industry level, e.g., in the process of fabricating semiconductor devices--from waste sulfuric acid effluents occurring from the process of fabricating semiconductor devices such as LSIs and VLSIs. It is here noted that this patent application is now laid open for public inspection under JP-A-3-303422.
This sulfuric acid recovery process is characterized in that waste sulfuric acid is fed to a cathode chamber of a multi-chamber type electrolyzer partition by at least one anion exchange membrane and at least one cation exchange membrane into three or more chambers, said cathode chamber being formed by the anion exchange membrane and the wall of the electrolyzer, or to a cathode chamber of a two-chamber type electrolyzer partitioned by an anion exchange membrane, thereby electrolyzing the sulfuric acid in an intermediate chamber formed by the anion and cation exchange membranes or in an anode chamber of the two-chamber type electrolyzer partitioned by a cation exchange membrane, so that the sulfuric acid can be concentrated with the generation of oxidizing substances. The regenerated sulfuric acid, because of containing oxidizing substances such as peroxomonosulfuric acid, peroxodisulfuric acid and hydrogen peroxide, can be reused at the steps of stripping and washing resists with no need of adding any fresh hydrogen peroxide solution.
However, a grave problem with currently available anion exchange membranes based on fluorine or hydrocarbons is that their acid resistance is low; that is, the concentration of sulfuric acid used therewith is limited to 50% by weight at most and preferably to the range of 10 to 30% by weight. Another problem is that the selective transmission ratio of sulfuric acid ions and hydrogen ions, viz., SO.sub.4.sup.2- /H.sup.+, is as low as 0.1 to 0.4 at a sulfuric acid concentration of 30 to 50% by weight.
The recovery process mentioned above makes it possible to recover sulfuric acid of purity high-enough to be used at the electronics industry level, but involves economical difficulty because of needing a number of expensive ion exchange membranes, anodes and cathodes.
Moreover, the concentration of sulfuric acid generated in the first-stage electrolyzer is limited to 30 to 50%. To concentrate this sulfuric acid and refine the oxidizing substances, previously refined sulfuric acid must be fed to the anode chamber partitioned by the second-stage ion exchange membrane. However, this must be done with a number of costly ion exchange membranes, anodes and cathodes.
One object of the present invention is to recycle sulfuric acid in a closed system by subjecting an inclusions-containing waste sulfuric acid effluent generated from the process of fabricating semiconductor devices such as LSIs and VLSIs to a relatively simple step without recourse to a number of electrolyzers and a number of ion exchange membranes, thereby recovering a sulfuric acid product containing sulfuric acid of purity high-enough to be reused in the process of fabricating semiconductor devices at the electronics industry level as well as oxidizing substances.
Another object of the present invention is to provide a process for recycling a waste sulfuric acid effluent with built-up impurities, which is discharged from a closed system in the form of discard, or for recovering a sulfuric acid product with reduced impurities.